1. Field of the Invention
The present invention relates to an electronic device capable of regulating a power switch to dissipate power, and more particularly, to an electronic device capable of avoiding abnormal operation due to overheating of a power switch.
2. Description of the Prior Art
Nowadays, there are a variety of electronic devices. For normal activation and operation, an electronic device needs to undergo a turn-on process. After power-on, power switches of a power supply device in the electronic device are first turned on, to transfer power of a primary power source to each circuit element in the electronic device. The power switches utilized for controlling power transferring are often realized by field-effect transistors (FETs) or bipolar junction transistors (BJTs). According to operation principle of an FET, current conduction between a drain and a source is controlled by a gate voltage of the FET. In a normal situation, after the power switches are turned on, each circuit element can start to operate. However, there is an overheating issue if FETs are applied as power switches.
Please refer to FIG. 1A, which is a schematic diagram of a conventional electronic device 10. The electronic device 10 includes a power supply device 100, a transistor M1, a capacitor C1 and a load LOAD1. The transistor M1 is an N-TYPE FET and acts as a power switch. When a gate voltage of the transistor M1 shifts from a low-level voltage to a high-level voltage, the current provided by the power supply device 100 can flow from a drain to a source, to charge the capacitor C1. Noticeably, at the moment that the drain and the source of the transistor M1 are conducted, a voltage across the capacitor C1 is around 0 volt, such that the source voltage of the transistor M1 is around 0 volt as well. Since the drain voltage of the transistor M1 substantially equals an output voltage of the power supply device 100, voltage difference between the drain and the source reaches maximum at the moment that the transistor M1 is turned on. Meanwhile, since the conduction current of the transistor M1 increases significantly, the transistor M1 has a great voltage difference and a great conduction current at the same time. According to operate principles of semiconductors, thermal power released by the transistor M1 substantially equals a product of the voltage difference between the drain and the source and the conduction current. Therefore, when a great voltage difference and a great conduction current exist at the same time, the transistor M1 instantly releases a great amount of thermal energy, causing the transistor M1 to activate overheating protection mechanism due to overheating, which protects the transistor M1 by automatic shut down, but the transistor M1 may have been burnt out by overheating.
Please refer to FIG. 1B, which is a time distribution diagram of the voltage drop, the conduction current and the thermal energy of the transistor M1 shown in FIG. 1A at power-on. After the electronic device 10 is turned on, the source voltage of the transistor M1 increases from 0 volt to the voltage provided by the power supply device 100 gradually. On the other hand, the drain voltage of the transistor M1 substantially equals to the output voltage of the power supply device 100 before the electronic device 10 is turned on, and the voltage difference between the drain and the source of the transistor M1 decreases gradually after the electronic device 10 is turned on. In addition, the current flowing through the transistor M1 increases rapidly from 0 A to a maximum value IMAX at a time TA. As mentioned before, after the transistor M1 is conducted, the transistor M1 includes a great voltage difference between the drain and the source and a great conduction current at the same time. In other words, the transistor M1 instantly releases a great amount of thermal energy around the time TA, causing the transistor M1 automatic shut down due to overheating, or immediately burned out.
Therefore, at the moment that the electronic device is turned on, the voltage difference between the drain and the source of the FET acting as a power switch is great. If the current flowing through the FET increases to a high level at the same time, the FET would instantly release too much thermal energy, which overheats the FET, such that the thermal shutdown mechanism is activated, or power switch is immediately burned out, causing the electronic device incapable of working normally.